Unless your bunk bed frame is full-size or an extra-long length, the mattress that fits the bunk is the same length and width as a twin mattress.
But, there are differences. Sometimes, especially for the top bunk, a true bunk-bed mattress may be thinner than a similar twin mattress.
Size of Mattresses
Basic bunk beds - two identical, narrow beds stacked one above another -- are sold in standard twin-bed size. Both twin and basic bunk-bed mattresses measure 39 inches wide, 75 inches long,
Thickness of Mattresses
Even though the length and width are the same as twin mattresses, a bunk-bed mattress may be a bit thinner, potentially at less than 6 inches thick.
While you could put a regular twin mattress on either bunk, provided the bunks are twin size, the thickness of the mattress also determines how much headroom is available between the bottom and top bunks as well as the top bunk and the ceiling.
A regular twin innerspring mattress may be a bit sturdier and thicker.
Atypical Bunk Bed Configurations
Full Bottom/Twin Top Some bunk beds feature a full-size bed on the bottom, with a twin bed on top. In this case, the bottom mattress is a standard full or double, measuring 54 inches wide and 75 inches long.
Twin XL Bunk beds designed to fit teens and adults may be a longer size, called a twin XL. These measure 39 inches wide, 80 inches long -- the same length as a king-sized bed.
Only use the mattress size designated for your bunk beds; do not use a twin mattress on a twin XL bunk frame, for instance, as the extra space between the mattress and frame may pose a safety hazard.